The invention relates to a system for imparting precisely controlled spin to a piston of a dead weight piston pressure measurement/calibration device without unpredictably disturbing its free movement.
Dead weight piston pressure measurement/calibration devices are well-known. Such devices ordinarily include a piston supporting a selected number of calibration weights. A "dead weight piston assembly" includes the piston, a bell housing, and the calibration weights. The piston is slidably disposed in a cylinder, in very low frictional relationship to the cylinder. Fluid, which can be gas or liquid, the pressure of which is to be defined, is metered into the bottom of the cylinder so as to push the dead weight piston assembly upward. When the force produced on the bottom of the piston by the pressurized fluid equals the weight of the dead weight piston assembly, the dead weight piston assembly "floats" in an equilibrium condition.
In the closest prior art dead weight piston calibration devices, the piston usually supports a hollow, partially cylindrical bell housing that encloses the cylinder within which the piston is disposed. A horizontal annular flange or ledge is attached to and extends outwardly from a lower outer surface portion of the bell housing. The piston, bell housing, and annular weight assembly are very precisely and symmetrically shaped and balanced. A selected number of calibration weights are stacked on the ledge for the purpose of precisely establishing the total weight of the dead weight piston assembly. A spin is imparted to the dead weight piston assembly, which is sufficiently symmetrical about the vertical axis of the piston that the piston spins freely within the cylinder, the outer surfaces of the piston being separated and lubricated from the walls of the cylinder by a thin layer of fluid, which can be a gas or liquid. The known weight of the dead weight piston assembly and the known effective area of the "piston-cylinder" produce a precise indication of the pressure being supplied to support the dead weight piston assembly in a "free-floating" condition between upper and lower stops of the dead weight piston calibration device.
Those skilled in the art know that the best accuracy can be obtained in a dead weight piston pressure measurement system while the dead weight piston assembly is rotating in a free-floating equilibrium condition in a particular rotation rate range, with no interference from a rotational drive system or a rotation measuring device.
Prior dead weight piston pressure measuring systems typically impart spin to the piston assembly by means of an elliptical pulley driven by a constant velocity belt. Or, a flexible member momentarily engages the dead weight piston assembly to accelerate it, but also causing vertical "bouncing" of the piston which results in pressure perturbations. The times at which this happens are unpredictable. Consequently, the measured pressure data contains corresponding unpredictable perturbations. Other prior systems reduce the pressure beneath the free floating rotating piston to lower it enough to cause it to engage a drive clutch and accelerate the piston rotation rate to the desired maximum value. Other prior systems are completely manual, depending on operator observation of piston rotation rate and manual turning on of the drive motor to accelerate the piston. These prior systems are not well suited to rapid, accurate, automated pressure measurement/calibration operations.
The centering of the piston in the cylinder is influenced by the rotation speed of the piston. Consequently, there is a slight dependance of the effective area of the piston-cylinder on piston rotation speed. In order to improve the reproduceability of a dead weight piston measurement, it is necessary to maintain the rotation speed of the piston within defined limits.
Thus, there is an unmet need to provide a drive system with a means of maintaining the rotation speed of the piston within predetermined range.
A problem with such prior dead weight piston calibration devices is that it previously has not been possible to impart adequate spin to the dead weight piston assembly without causing piston motion perturbations due to permanent or momentary contact of the piston with the piston drive mechanism. Such perturbations obviously reduce the accuracy of the pressure measurement or calibration being performed. In prior dead weight piston calibration devices in which the piston drive system automatically engages and accelerates rotation of the piston, the precise moment or instant of engagement are indeterminate, so the moment or instant of occurrence of the above mentioned perturbations also are indeterminate, and there is no basis for reviewing when the pressure data is inaccurate because of the perturbations.
Accordingly, there is an unmet need for an improved dead weight piston pressure measurement or calibration device that is more accurate than those of the prior art.
More specifically, there is an unmet need to eliminate inaccuracy in a dead weight piston measurement or calibration device due to "random" vertical force components applied to a piston thereof in the course of causing the piston to rotate while in a free-floating condition.
Another unmet need of the art is to eliminate unpredictability of the times at which the piston drive mechanism engages the rotating piston assembly to accelerate it.